330 ZOOLOGY SECT. 



a share in the formation of the mesoderm. Two lateral pouch - 

 like diverticula of the endoderm sac give rise to the mesoderm ; 

 in these a cavity appears the rudiment of the ccelome. A 

 second pair of sacs of the same character becomes developed 

 further back. A large pra3-oral lobe is formed, and the anus 

 makes its appearance at a little distance behind the mouth as an 



FK;. 'Jd4. Phoronis, Development. A, young larva ; B, larva after the formation of the post- 

 oral circlet of tentacles ; C, larva with commencing pit-like involution ; I), larva with invagina- 

 tiou partly everted ; E, invagination completely everted, -m. mouth ; an. anus ; ic. involution 

 to furni the body. (From Balfour's Embryology.) 



invagination of the ectoderm ; it becomes surrounded by a circlet 

 of cilia (Fig. 264, A). The part of the body on which the anus 

 is situated becomes elevated into a conspicuous process. Behind 

 the mouth there is a circlet of cilia and from this region grow out 

 a circlet of processes the rudiments of the larval tentacles (B). 

 The larva has now reached the stage to which the term Actino- 

 trocha is applied. It has a large hoodlike lobe overhanging the 



